Conventional air separation units (ASUs) for the production of nitrogen, oxygen, and argon using cryogenic distillation technology are well known. ASUs typically separate air into its primary component gases at very low or cryogenic temperatures using one or more distillation columns. It is essential that certain impurities such as water vapor, carbon dioxide, nitrogen oxides, and trace hydrocarbons be removed from the compressed air feed prior to cryogenic distillation to avoid freezing of the impurities in the cryogenic equipment and potentially causing explosion. Any freezing will require stopping the process to remove the detrimental solid mass of frozen gases which is costly and can damage equipment. Generally, the content of water vapor and carbon dioxide in the compressed air feed stream must be less than about 0.1 ppm and about 1.0 ppm, respectively in order to prevent freeze up of these gases in an ASU.
The air feed stream is therefore cleaned or purified to remove these impurities prior to distillation typically by an adsorption process employing two or more vessels filled with beds of one or more adsorbents which selectively adsorb the impurities. Once an adsorption bed is saturated with impurities, it needs to be regenerated by removing the impurities so the bed is ready for further use.
Current commercial methods for this pre-purification of air generally include either one of or a combination of a cyclic pressure swing adsorption or temperature swing adsorption process. Pressure swing adsorption (PSA) uses a change in pressure, including vacuum, to regenerate the adsorbent and temperature swing adsorption (TSA) uses a thermal driving force such as a heated purge gas to desorb the impurities. The TSA process usually requires much lower amount of purge flow compared to PSA and affords a longer cycle time, typically in the range of 4 to 10 hours. The PSA process requires a greater amount of purge flow and affords a much shorter cycle time in the order of minutes. Moreover, there is no requirement for regeneration heat energy in PSA as opposed to TSA. Hence, when there is sufficient waste nitrogen available in a cryogenic air separation plant, the PSA process is usually a preferred option for air prepurification due to its simplicity, lower capital cost, and lower operating cost.
One disadvantage of the PSA process is that the adsorbents do not always get completely regenerated at the completion of the purge step and hence their dynamic capacity, the ability to remove the desired components, is lowered compared to the adsorbents regenerated in TSA processes. As a result, the PSA process is typically run for short cycle times necessitating that the bed undergoes blowdown (vent) and repressurization at fairly frequent intervals. During the blowdown step, there is a noticeable loss of air trapped within the void spaces of the vessel and piping as well as the air adsorbed on or within the adsorbents. This air loss, referred to by various terms such as blowdown loss, vent loss, or bed switch loss, represents a significant waste as the air is not utilized towards air separation downstream of the prepurifier. More significantly, there is an operational cost disadvantage as the air lost during bed switches utilizes valuable compression power. Accordingly, there is an increasing need to reduce this power requirement and increase the operational efficiency of the PSA prepurification process.
One way to lower the power requirement of the PSA process is to reduce the blowdown or bed switch loss described previously. This can be accomplished by reducing the frequency of bed blowdown and repressurization, for example by extending the cycle time for which the bed is kept online prior to being switched to regeneration. However, since the conventional commercial adsorbents, including zeolite-alumina composites, afford only modest dynamic working capacities for removal of the common air contaminants described above, an increase in cycle time would require either reducing the feed flow significantly at a fixed bed size or require a drastic increase in the bed size at a fixed feed flow rate. However, it has been found that by modifying the adsorbents employed to provide increased working capacities the improvements required can be achieved.
One strategy to increase the mass transfer rate of an agglomerated adsorbent particle is to reduce the particle size of adsorbent aggregates such as that disclosed in U.S. Pat. No. 6,790,260. This will increase the adsorption and/or desorption kinetics by reducing the path length needed for adsorbates to travel through the rate-limiting macropores of the agglomerated adsorbent. Reducing the particle size, however, has its limitations: higher pressure drop and increased risk of fluidization in unconstrained adsorption beds quickly become issues for adsorption process and system designs. Moreover, containment and manufacturing of small particle sized agglomerates represent other drawbacks that need to be resolved.
Another strategy to increase the mass transfer rate is to formulate the adsorbent as a composite adsorbent by mixing zeolite and alumina. Such composite adsorbents are disclosed in U.S. Pat. Nos. 5,779,767; 6,027,548; 6,358,302; 6,638,340; and 8,65,7924.
Yet another strategy to increase mass transfer rate can be to formulate an adsorbent in the form of a core-in-shell adsorbent. For example, Lu et al (EP1080771; U.S. Pat. No. 6,284,021) described a core-in-shell composite adsorbent with an inner core comprising a non-porous and non-adsorbent material and at least one outer layer comprising an adsorbent material forming a shell. Such composite beads having different ratios of inner radius to outer radius were disclosed to improve the mass transfer in adsorption processes without reducing the particle size of the beads. No experimental data was reported. However, benefits of using a 4 mm diameter composite bead over a conventional 2 mm diameter adsorbent bead were estimated by simulating production of 55 metric tons of oxygen product from air in a VSA plant. Composite beads with a non-porous core have a lower active adsorbent content in comparison to homogeneous adsorbent beads therefore resulting in lower overall capacity, which however is offset by the improved mass transfer rate.
Brandt et al. published results of O2-VPSA pilot plant tests using core-in-shell adsorbents (80wt % NaX zeolite, 20wt % attapulgite clay as shell) in Adsorption 13: 267-279, 2007. Table 4 in this publication lists core-in-shell adsorbents wherein the shell contains NaX zeolite and attapulgite clay in an 80/20 wt % ratio and wherein the core is a solid inert core of the formed-glass granules or quartz sand type. Sample 1 in Table 4 is a comparative example of a granule without any core present. Sample 1 is described under section 4.2 as a “state of the art full body NaX zeolite”. In column 7 of Table 4, the volume-related productivity at 90% purity is provided for the comparative sample 1 and for the core-in-shell compositions. From these data, it is clear that the volume-related productivity is highest for the comparative sample 1 and lower for all of the core-in-shell compositions (samples 2-11). Thus, the comparisons in Table 4 and discussion in section 4.2 of the publication indicate that an adsorbent bed of a given diameter and height using conventional adsorbent beads treats higher feed flow than an adsorbent bed containing any of the core-in-shell adsorbent beads. The publication further discloses that the core-in-shell adsorbent shows improved performance only when the total amount of active adsorptive material in the bed is used as the basis (column 8, zeolite-content related productivity).
Gerds et al. (EP2198946; U.S. Pat. No. 8,814,985) disclosed another type of core-in-shell composite adsorbent, having a porous and non-adsorbent core and a porous and adsorbent shell. The porous cores are taught to be made up of agglomerates formed from 0.01 to 5 μm particles of inorganic material containing hydroxyl groups, wherein the mean particle size of agglomerates is equal to or smaller than the mean particle size of agglomerated adsorbent particles in the shell. Adsorbent beads containing both porous core and porous shell are shown to exhibit higher crush strength than adsorbent beads containing a non-porous core and a porous shell. Results of pilot testing of core-in-shell composite adsorbent beads in a hydrogen PSA process to produce high purity hydrogen product, in a TSA process to remove nitrogen impurity from helium, and in a PVSA process for producing 90+/−0.5% purity oxygen product from air are reported that indicate improved performance over that of conventional homogenous adsorbent beads.
There is a continuing need for superior volumetric performance and attrition resistant adsorbents for use in PSA prepurifiers in cryogenic air separation applications. Volumetric performance of an adsorbent can be calculated by dividing the volumetric flow rate of feed gas at standard conditions of temperature and pressure by adsorber vessel volume occupied by adsorbent bed. For a fixed bed size (adsorber vessel volume occupied by adsorbent bed) the operators can easily rank various adsorbents based on experimental data or simulation results of feed flow rates. Another performance indicator commonly used by plant operators is called bed-size factor, which is calculated by dividing feed flow rate by cross-sectional area of adsorbent bed. Ranking of adsorbents based on this indicator assumes each adsorbent is loaded in the adsorber vessel to same height. A suitable adsorbent must not only produce purified air stream, but indeed it must also have acceptable physical properties. Physical integrity of the adsorbents is an important commercial consideration in their selection. In PSA and TSA systems, cyclic process conditions may cause adjacent adsorbent particles to contact and abrade each other. Particles can experience surface attrition and/or break up into smaller particles. This can result in higher pressure drop, loss of adsorptive material from the adsorbent bed, as well as plugging or other problems in downstream equipment. Since the active adsorptive material of a core-in-shell adsorbent resides in the shell, attrition resistance property of the adsorbent is a much stronger consideration in their selection for use in commercial scale prepurifiers.